As compared with a CRT (cathode ray tube), a liquid crystal display (LCD) has great advantages such as thin shape, light weight, low power consumption. The liquid crystal display generally comprises a liquid crystal cell and a pair of polarizing plates provided on both sides of the cell. The liquid crystal cell comprises rod-like liquid crystal molecules, a pair of substrates between which the rod-like liquid crystal molecules are provided, and an electrode layer having a function of applying a voltage to the rod-like liquid crystal molecules. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecules. In addition, an optical compensatory film (phase retarder) is often provided between the cell and the polarizing plate to remove undesirable color from an image displayed on the liquid crystal cell. A laminate of the polarizing plate (polarizing membrane) and the optical compensatory film serves as an elliptically polarizing plate. In some cases, the optical compensatory film has a function of enlarging a viewing angle of the liquid crystal cell. As the optical compensatory film, a stretched birefringent film has been conventionally used.
In place of the stretched birefringent film, an optical compensatory film comprising an optically anisotropic layer composed of discotic compound has been proposed (for example, in Referential Patent Publications 1 to 4). In the optically anisotropic layer, molecules of the discotic compound are aligned, and the alignment is fixed. The discotic compound generally gives large birefringence. Further, the molecules of the discotic compound can be oriented according to one of various alignment modes. Therefore, an optical compensatory film obtained from the discotic compound can have specific optical characters that cannot be given by the conventional stretched birefringent film.
A liquid crystal display having a liquid crystal cell of bend alignment mode, in which rod-like liquid crystal molecules in upper parts and ones in lower parts are essentially reversely (symmetrically) aligned, has been proposed (for example, in Referential Patent Publications 5 and 6). Since the liquid crystal molecules in upper parts and those in lower parts are symmetrically aligned, the liquid crystal cell has self-optical compensatory function. This mode is, therefore, also referred to as OCB (optically compensatory bend) mode. In addition to the self-optical compensatory function, the liquid crystal display of OCB mode has an advantage of rapid response.
If equipped with a liquid crystal cell of hybrid alignment mode, a liquid crystal display of reflection type can have the same self-optical compensatory function as the display of bend alignment mode. The hybrid alignment mode is also referred to as RAN (hybrid-alignment-nematic) mode.
The liquid crystal display of bend alignment or hybrid alignment mode has a wide viewing angle and responds rapidly, as compared with popular displays (of TN and STN modes). Nevertheless, in consideration of images given by CRT displays, it is still necessary to improve the display of bend alignment or hybrid alignment mode. In the display of bend alignment or hybrid alignment mode, as well as in the popular display, an optical compensatory film can be used to improve qualities of displayed images. However, a conventional stretched birefringent film cannot fully fill the role of the optical compensatory film when used in bend alignment or hybrid alignment mode. As described above, it has been proposed to use an optical compensatory film comprising a transparent support and an optically anisotropic layer of discotic compound in place of the stretched birefringent film. Further, a liquid crystal display of bend alignment mode equipped with an optical compensatory film containing discotic liquid crystal compound is also proposed (for example, in Referential Patent Publications 7 and 8). The optical compensatory film containing discotic liquid crystal compound remarkably improves the viewing angle of liquid crystal display of bend alignment mode.
However, it is reported (for example, in Referential Patent Publication 9) that images given by the liquid crystal display of bend alignment mode equipped with the optical compensatory film containing discotic liquid crystal compound are often colored with leaked light of a specific wavelength. According to the publication, this undesired coloring is due to wavelength dependence of the transmittance of the elliptically polarizing plate (i.e., laminate of the polarizing membrane and the optical compensatory film). It is also reported that, if the optically anisotropic layer and the polarizing membrane are placed so that an average of directions obtained by projecting normal lines of discotic planes of the discotic compound molecules onto the anisotropic layer may be essentially at 45° to the transmission axis in the plane of the polarizing membrane, the liquid crystal cell of bend alignment mode is maximally optically compensated.
In order to reduce change of hue and to prevent inversion of gradation, various methods are proposed (for example, in Referential Patent Publications 10 and 11) for the liquid crystal cell of bend alignment mode equipped with the optical compensatory film containing discotic liquid crystal compound.
Referential Patent Publication 1: Japanese Patent Provisional Publication No. 6(1994)-214116
Referential Patent Publication 2: U.S. Pat. No. 5,583,679
Referential Patent Publication 3: U.S. Pat. No. 5,646,703
Referential Patent Publication 4: German Patent Publication No. 3,911,620 A1
Referential Patent Publication 5: U.S. Pat. No. 4,583,825
Referential Patent Publication 6: U.S. Pat. No. 5,410,422
Referential Patent Publication 7: Japanese Patent Provisional Publication No. 9(1997)-197397
Referential Patent Publication 8: International Publication No. 96/37804
Referential Patent Publication 9: Japanese Patent Provisional Publication No. 11(1999)-316378
Referential Patent Publication 10: Japanese Patent No. 3,056,997
Referential Patent Publication 11: Japanese Patent Provisional Publication No.2002-40429